This invention relates to rear projection screens for use in displaying an image projected from an image source. More particularly, the invention relates to a rear projection screen which utilizes a front lenticular lens array to distribute light from the image source, and a rear Fresnel lens to receive light from the image source and to collimate this light for the lenticular lens array.
Rear projection screens are used in a variety of applications, such as television, video games, film media displays, microfilm readers, radar and avionic displays, flight simulators and traffic control lights. In such applications, an image behind the screen is projected forward along a central projection axis to the screen, which displays the image to viewers located in front of the screen.
A problem encountered in these rear projection systems is that the light energy tends to be concentrated along the projection axis. This is particularly troublesome in those applications in which a room-sized audience is contemplated, such as, for example, projection television. In such cases, the angle of view will normally vary little in the vertical direction, but widely in the horizontal direction.
It is an objective of the screen designer to achieve uniform illumination of the screen, and also to distribute the screen image over as wide a horizontal viewing angle as possible.
In U.S. Pat. No. 4,573,764, assigned to the same Assignee as this Application, a rear projection screen is described which achieves a remarkably wide horizontal angle of view of greater than plus or minus 85.degree. relative to the projection axis, with the luminance at any horizontal viewing angle being no less than 40 percent of the peak luminance. This is accomplished by combining a conventional light collimating circular Fresnel lens with a light distributing lenticular lens array in which the individual lenticular elements have critical design parameters including height-to-width and overall width-to-tip width ratios, as well as tip regions characterized by two lateral convex and central concave cylindrical elements, resulting in exceptional horizontal dispersion of light, while at the same time exhibiting minimal color shift.
Disperson of the light rays in the vertical direction is generall accomplished by diffusion means located between the light collimating Fresnel lens and the light spreading lenticular lens array. Such diffusion means also tends to reduce color shift by achieving a more even horizontal dispersion of the light rays.
In the case of projection television, it is usually not necessary to distribute the light through a large vertical included angle, since the individual vertical viewing angles within the audience typically vary only a few degrees from one another. On the other hand, it is advantageous to reduce the vertical included angle of the audience field as much as possible, since this angle is approximately inversely proportional to the screen gain, defined as the amount of front screen brightness (output) obtained for a given amount of rear screen illumination (input).
However, in a projection television system employing a wide angle screen of the type described, when the diffusion means is changed to reduce the vertical viewing angle, a bright band appears horizontally across the screen, becoming increasingly apparent as the vertical audience field is further reduced, and moving in tandem with vertical shifts in the viewer's head.
Accordingly, it is an object of the invention to reduce or eliminate this bright band from wide angle rear projection screens of the type described.
It is another object of the invention to improve the luminance uniformity of such rear projection screens.
It is another object of the invention to produce such a rear projection screen having a reduced vertical viewing angle.
It is still another object of the invention to increase screen gain without the onset of the bright band.